1. Field of the Invention
This invention relates to servo valve systems each including a pilot servo valve assembly and at least one fluid pressure servo valve assembly driven by such pilot servo valve assembly, and more particularly to a fluid pressure servo valve assembly having particular utility in equipment, e.g. a vibration table, requiring a servo valve system that can handle a fluid flow of a large capacity with a high responsiveness.
2. Description of the Prior Art
Heretofore, research into servo valve systems capable of handling a fluid flow of a large capacity (high flow rate), such as 5,000 l/min., has not been popular because of the fact that there is not much commercial value in these systems. Although these systems have had certain applications, what is required in these applications is merely determination of the flow rate, and the requirement for the response is very low, or the limits placed on the operation are several hertz, for example.
However, there has in recent years grow a strong need to cause colossal equipment, equipment for nuclear reactors weighing several thousand tons, for example, to vibrate in order to verify the capability of such equipment to withstand the shock of earthquakes. Thus there has been created a demand for servo valve systems of a large capacity and high responsiveness for effecting control of the operation of causing such equipment to vibrate.
In one system of the prior art to cope with this situation, hydraulic pressure is increased in two or three stages, and a main servo valve assembly including a spool valve member of a large diameter (which is referred to as a power or main spool valve member) of the last stage is driven so as to effect control of a high flow rate. In this type of system, there are provided a pilot servo valve assembly in addition to main servo valve assembly driven by the pilot servo valve assembly. The main servo valve assembly comprises a valve body formed therein with a cylindrical bore, an input port and output ports opening in the cylindrical bore, and a power spool valve member fitted in the cylindrical bore for axial reciprocatory movement. The input port is connected and disconnected with the output ports as the power spool valve member moves in the cylindrical bore for axial reciprocatory movement. The main servo valve assembly further comprises two pressure chambers defined by inner faces of end walls of the valve body and axial end faces of the power spool valve member, and passages for communicating the two pressure chambers with the pilot servo valve assembly. An actuating pressurized fluid introduced from the pilot valve assembly alternately into one or the other of the two pressure chambers acts on the axial end faces of the spool valve member to move the same in axial reciprocatory movement in the cylindrical bore.
In this type of system of the prior art, the passages communicating the pilot servo valve assembly with the two pressure chambers have a large length because the pressure chambers are disposed on the axial ends of the cylindrical bore formed in the main servo valve assembly. This means that the volume of the passages is increased, so that a delay is caused to the transmission of pressure signals from the pilot servo valve assembly to the two pressure chambers. Thus the main servo valve assembly becomes low in responsiveness and is unable to operate in a suitable manner to handle high frequencies.
Moreover, the great length of the fluid passages of the pressurized fluid inevitably renders the construction of the flow passages complex, and the fluid passages must be formed with a number of holes for machining the valve assembly, with the air being collected in such holes and making it impossible for the valve assembly to achieve the desired performance.